The mixer-settler process consists in producing a suitably fine dispersion of two immiscible liquids in one another for facilitating exchanges therebetween, and then in re-separating the two immiscible liquids by settling. In general, one of the liquids is constituted by an impure solution of a given substance, while the other liquid is an organic solvent serving either as a solvent which is specific to the substance in question, or else as a dilution medium for an ion exchanger specific to the substance in question. The complete installation generally comprises an extraction battery constituted by juxtaposing a plurality of mixer-settlers in which the impure solution of the substance under consideration flows as a counter-current to the organic liquid, followed by an identical type of re-extraction battery in which the organic liquid flows as a countercurrent to a pure solution which picks up the substance, thereby enabling the organic liquid to be used in a closed circuit.
Each mixer-settler in the battery is essentially constituted by a mixer which receives the two basic liquid substances and which stirs them together in order to obtain the required dispersion, followed by a settling tank through which the mixture flows slowly while progressively separating out by virtue of the liquids being immiscible and the dispersion being unstable. Clearly, for a given throughput, the bulk of each mixer depends on the length of time the dispersion remains therein before moving on to the settler, and this length of time depends on the efficiency of the exchanges, which efficiency increases with increasing fineness of the dispersion. Similarly, the bulk of each settler depends on the length of time it is occupied by the dispersion passing therethrough, and this length of time is a function of the speed at which the dispersion coalesces, and the finer the dispersion, the slower the speed at which it coalesces. Consequently, there is always an overall optimum size for the droplets in the dispersion which provides minimum bulk and thus minimum cost for any given installation.
In addition to the mixer and the settling tank, the liquids and the mixtures of liquids are moved through the installation by means of pumps or by means of stirring pumps.
Accompanying FIG. 1 is a diagram of a mixer-settler of the type to which the present invention is applicable. In this figure:
a mixer 1 has liquid inlets 2 and 3 and a stirrer 4;
a pump 5 raises the mixture made in the mixer to a suitable level;
a chute 6 conveys the liquid mixture leaving the pump 5 to the inlet to the settling tank 7 in which the two liquids flow at slow speed and separate progressively in order to give rise firstly to a light phase (generally organic) which is removed at 8, and secondly to a heavy phase (generally aqueous) which is removed at 9; and
a regulator 12 sets the level of the interface between the two phases.
It may be observed, and this is one of the essential characteristics of installations of the type described above and consequently of installations to which the present invention is applicable, that the settling tank 7 is fed from the end thereof (called the feed end) which is furthest from the mixer, and as a result:
firstly a chute 6, which may be very long, must be provided in order to convey the mixture of liquids to the feed end of said tank; and
secondly the liquid flow direction through the settling tank 7 takes place towards the pump 5 which feeds the chute 6; the liquid separated in the settling tank thus leaves said tank from the face thereof which is closest to said pump 5 (or from the immediate vicinity of said face).
Such installations may be referred to as "reverse liquid flow" installations (since the liquid flow direction in the settling tank takes place towards the pump which feeds said settling tank), and they constitute the type of installation to which the present invention is applicable. In such installations, the settling tank thus has a liquid "inlet face" which, in conventional installations, is the face furthest from the feed pump, and a liquid "outlet face" for liquids that have settled, which face is closest to said pump.
Designing and using installations of this type shows:
firstly, given the flow of the liquids along the chute and the length of said chute, that a degree of liquid separation generally takes place in said chute, and if said liquids are not to be re-mixed on insertion into the settling tank, special precautions must be taken at the end of the chute; and
secondly, in large installations, that the construction and support of the chute give rise to certain difficulties.